The manufacture of glass windows coated with infrared radiation reflective films is known in the art, see U.S. Pat. No. 4,440,822, issued Apr. 3, 1984, entitled "Non-Iridescent Glass Structures". The purpose of placing such a coating on a glass is that the coating is effective in reflecting radiation in the infrared band. This radiation is heat radiation and if it is trapped within a building by the glass structure, the building requires less total energy to keep it heated to a particular temperature.
We are also aware of U.S. Pat. No. 4,144,362, issued Mar. 13, 1979, for "Method for Coating Glass Using Monoalkyltin Trihalides". This patent discloses a method for obtaining a stannic oxide coating on a glass surface by applying an organotin compound to a heated glass surface. The purpose of coating the glass was to improve the impact and abrasion resistance of the glass. This patent disclosed that a butyltin trichloride could be pyrolized to provide an acceptable stannic oxide coating on heated glass surfaces. As described in the patent, the stannic oxide coating in combination with a synthetic polymer coating at the cold end of the annealing lehr improved the scratch resistance of the glass article. In particular, the patent also indicated that the containers coated with this material exhibited a higher burst strength than containers coated using other prior art organotin compounds such as dimethyltin dichloride. Once again, the aforementioned '362 patent is devoid of mentioning the utilization of the butyltin trichloride in order to make infrared reflective glass sheets, and does not mention doping the film with fluoride ions.
In our opinion, it is desirable to form an infrared reflective film on a glass surface using butyltin trichloride because this material has several advantages. These advantages are:
1. The utilization of butyltin trichloride results in the application of a very smooth film on a glass surface. A smooth film is desirable because it reduces the variation in reflected color and improves the overall reflective color appearance, gives less film reflected color "texture" or "mottle" and gives a smoother reflective color variation across a glass sample than other organotin compounds such as dibutyltin diacetate (DBDA) or dibutyltin difluoride (DBDF).
2. Butyltin trichloride, especially n-butyltin trichloride (NBTC) is very soluble in a water-miscible solvent such as methanol. This allows a wide range of organotin compound concentration solutions to be used as well as allowing doping with water soluble fluoride compounds such as ammonium fluoride. The high concentrations of butyltin trichloride possible allows rapid formation of relatively thick films (150 to 1000 nanometers) at high volume glass throughout with minimum cooling of the glass substrate due to solvent evaporation and limited spray zone length.
3. Butyltin trichloride does not hydrolyze in water like most inorganic tin chlorides, therefore allowing the use of water soluble fluorides such as ammonium fluoride as the fluoride dopant.
4. Butyltin trichloride allows formation of good infrared reflecting films without the use of organic solvents. This eliminates the need for costly hydrocarbon emission control equipment and the use of flammable or toxic solutions.
5. The combination of lower organic content and presence of chlorine in the butyltin trichloride solutions doped with fluorine improves the electroconductivity and infrared reflectance of the resulting film over other organotin compounds. The butyltin trichloride-induced film has a grain structure which gives improved electroconductivity.
6. The relatively high vapor pressure of butyltin trichloride at room temperature allows spraying on the glass ribbon with less cooling than most tin compounds. This high vapor pressure also allows vapor as well as solution spraying to form the films. In vapor spraying, the butyltin trichloride can be doped with fluorine-containing compounds such as 1,1,2, trichloro-1,1,2, trifluoroethane to obtain the proper level of fluoride doping.
7. The amount of fluoride ion doping in the butyltin trichloride solutions and the SnO.sub.x film to obtain good electroconductivity is much less than in other organotin solutions, such as those employing dibutyltin diacetate (DBDA) and dibutyltin difluoride (DBDF). This results in less fluoride emissions.
As an additional matter, when one is dealing with infrared reflective glass, very desirable infrared reflective properties may be achieved at relatively low thicknesses of films from butyltin trichloride as compared to those from other materials, for example, dibutyltin diacetate doped with fluoride ions. However, we found that when we applied butyltin trichloride directly to the surface of the glass sheet, there was the undesirable interaction of the chlorine in the spray material and the sodium on the surface of the glass sheet which resulted in the formation of crystals of sodium chloride. Formation of crystals resulted subsequently in the production of voids in the film and the light scattering defect known as haze. This haze is unacceptable because it is noticeable when looking through the glass.
We found that we could solve the problem of haze production by applying a seal coating on the surface of the glass sheet prior to the application of the coating of tin oxide formed by decomposition of butyltin trichloride. The special seal coating leads to formation of a tin oxide film on the glass having a columnar grain microstructure that gives special qualities to the tin oxide film.
No search was conducted on the subject matter of this specification in the U.S. Patent Office or in any other search facility. We are unaware of any prior art more relevant to the subject matter of this specification than that which was set forth hereinabove.